A power supply device (AC/DC converter) using a multi-resonant half-bridge switching circuit is known as a power supply for a thin television receiver or the like (see Patent Document 1). The configuration of a conventional general AC/DC converter 50 is shown in FIG. 10. In the AC/DC converter 50, first, a rectifying circuit 53 such as a diode bridge rectifies AC power from a commercial power supply 52 and a capacitor 54 smoothes the rectified power. Next, a multi-resonant half-bridge DC/DC converter performs a voltage conversion. The DC/DC converter includes a multi-resonant half-bridge switching circuit composed of two switching elements Q51 and Q52 connected to a primary coil Np50 of a transformer 51, and rectifying diodes D53 and D54 respectively connected to the secondary coils Ns51 and Ns52 of the transformer 51. A resonant circuit composed of an inductor Lr and capacitors Cv and Ci is connected to the primary coil Np50 of the transformer 51.
As the switching elements Q51 and Q52, e.g., MOSFETs are used and the switching elements Q51 and Q52 are alternately turned on and off after a predetermined dead-off time. The commercial power supply 52 for the AC/DC converter 50 has a frequency of 50 Hz or 60 Hz. However, it is possible to increase a frequency of a current inputted to the primary coil Np50 of the transformer 51 by shortening a period in which the switching elements Q51 and Q52 are alternately turned on and off as compared with a period of the commercial power supply 52.
The transformer 51, the inductor Lr or the like can be miniaturized by increasing a switching frequency of the AC/DC converter 50. Meanwhile, switching loss increases as the switching frequency rises, but in the multi-resonant half-bridge switching circuit, the switching loss can be suppressed by zero voltage switching (ZVS) or zero current switching (ZCS). Thus, the multi-resonant half-bridge switching circuit may be a circuit suitable for high frequency.
As described above, since the switching loss is small in the multi-resonant half-bridge switching circuit, the multi-resonant half-bridge switching circuit is used for a small-sized, high-efficiency power supply. In a case where circuit efficiency is further required in the high-efficiency power supply, main loss is limited to the diodes and the transformer since the loss involved in switching conditions is small. In particular, the loss of the diodes becomes bigger in proportion to an increase in load capacity and thus it is important to reduce the number of diodes.
[Patent Document 1] Japanese Patent Application Publication No. Hei 8-214559.